In a technique for a non-contact detection and measurement of a defect on the surface or in the inside of an object, such as a concrete or steel structure, a vibration is induced in the object by means of a vibrator, and a vibration analysis is performed by optically measuring the distribution of the amplitude of the vibration on the surface of the object (for example, see Patent Literature 1). An image obtained by the vibration analysis shows a discontinuity due to a defect inside the object, thereby enabling a non-contact detection of the inner defect.
Patent Literature 1 discloses a method for examining an object having a rough surface. In this method, the entire examination area on an object to be examined is illuminated with laser light produced by expanding a laser beam using a beam expander. The laser light is scattered on the rough surface, forming a light-and-dark pattern called the “speckle” due to the mutual interference of the scattered rays of light. This speckle pattern is made to interfere with a reference laser light branched from the illuminating laser beam, and an interference image is taken with a CCD camera or similar device. Two such images are respectively taken before and after the occurrence of a displacement on the surface of the object due to an externally given elastic wave. The distribution of the displacement on the examination area is calculated from those two images. By such a technique, the measurement of the entire examination area can be performed at one time.